PSI - Issue 23

A. Eremin et al. / Procedia Structural Integrity 23 (2019) 233–238 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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During stable crack propagation period, the texture becomes a rougher and a large number of secondary cracks are seen for both specimens. UFG Ti demonstrates more brittle fracture processes, the microcrack and stable crack growth stages are represented by smoother and finer microstructure in comparison with CG Ti (Fig. 5d,e). Prior to the final fracture of UFG specimens, the crack extension at individual cycles results in separate bands divided by striations which are easily seen on the fracture surface (Fig. 5f).

5. Conclusion

The significant difference in the deformation and fracture mechanisms of the CG and UFG technical grade titanium has been revealed under static and cyclic loading. It is shown that the grain size refinement results in the twofold increase of the tensile strength (from 419 to 863 MPa) and double reduction of strain at break (from 32% to 15%). The UFG Ti emits much more pronounces acoustic signal during fracture which being attributed to energy release from the dislocation bunches on the structural elements barriers. The larger area of grain boundaries in the UFG Ti suppresses spreading of plastic deformation and it localized in the narrow neck area, while CG has more uniform deformation pattern and the large area of plastic deformations. SEM fractography analysis has shown the difference in fracture surface patterns: the CG Ti exhibits uniform ductile fracture resulted from the normal tearing, whereas the UFG Ti fracture surface contains three various zones attributed to different failure processes (normal tearing, transient zone, and shear fracture). Crack growth kinetics of CG and UFG titanium specimens allow one to conclude the following:  at the crack nucleation stage presence of many grain boundaries in UFG Ti prevent the development of plastic deformations which would reduce local stresses and accumulated elastic energy in the vicinity of appeared microcracks, thus crack emerges on the surface of UFG Ti earlier than for CG Ti;  at the crack propagation stage, UFG Ti has lower crack growth rates due to grain boundaries and dislocations which in this case are obstacles for the transgranular type of fracture. Therefore a lot of structural defects in UFG Ti could have a negative effect on crack initiation at the first stage and then have a positive impact by suppression of crack growth processes at the second stage. Acknowledgements SEM fractography was performed by LEO EVO 50 microscope in “NANOTECH” of ISPMS SB RAS. The work was supported by President Scholarship for young scientists and graduate students in 2018-2020 (reference number SP-198.2018.4); RF President Council Grant for the support of leading research schools (reference number NSh-5875.2018.8.) Browne M., Gregson P.J., 2000. Effect of mechanical surface pretreatment on metal ion release. Biomaterials 21(4), 385 – 392. Ovid’ko I.A. and Langdon T.G., 2012. Enhanced ductility of nanocrystalline and ultrafine-grained metals. Reviews on Advanced Materials Science 30(2), 103 – 111. Valiev R.Z., Sabirov I., Zhilyaev A.P., Langdon T.G., 2012. Bulk Nanostructured Metals for Innovative Applications. The Journal of The Minerals, Metals & Materials Society 64, 1134 – 1142. Cavaliere P., 2009. Fatigue properties and crack behavior of ultra-fine and nanocrystalline pure metals. International Journal of Fatigue 31, 1476 1489. Fin tová S ., Arzaghi M. , Kuběna I ., Kunz L., Sarrazin-Baudoux C., 2017. Fatigue crack propagation in UFG Ti grade 4 processed by severe plastic deformation. International Journal of Fatigue 98, 187-194. Panin V.E. and Egorushkin V.E., 2009. Physical mesomechanics and nonequilibrium thermodynamics as a methodological basis for nanomaterials science. Physical Mesomechanics 12, 204-220. References